1. Field of the Invention
The invention in general relates to dielectric fluids for capacitors and more particularly to such fluids that are used in capacitors having metallized electrodes formed on plastic film.
2. Description of the Prior Art
The present invention relates to electrical capacitors which are constructed from a pair of metallized plastic film sheets. A thin layer of a metal, generally aluminum, is deposited on the surface of one side of each sheet and serves as an electrode. The plastic forms the dielectric. Preferably, the plastic is polypropylene, though other plastics, such as polyester, polycarbonate, polystyrene, polysulfone and polyethylene terephthalate may be used. Such capacitors are generally enclosed in a can which is filled with a dielectric fluid capable of operating under high voltage stress, as for example above 1350 volts/mil. The main problem in such capacitors is capacitance loss during operation. Some causes of capacitance loss are the presence of air or other gas between the electrodes and the absorption of dielectric fluid by the film which causes it to swell and lose dielectric strength. Air may be excluded from between the film layers by tightly winding the film and heat treating it. However, this does not entirely prevent dielectric fluid from entering the film roll. Also, gas may be generated by high stress electrical discharges in the capacitor. For these reasons, it is desirable that the dielectric fluid be highly viscous to impede its entering the rolled film, that its propensity to wet the film is low thus reducing spreading, that its tendency to swell the film is low, and that its capability of absorbing gas is high.
Glyceryl triacetate is known to be an excellent dielectric fluid in metallized film capacitors due to its relatively low wetting and swelling characteristics. See U.S. Pat. No. 4,459,637 issued to the present inventor. However, this fluid also has low viscosity and poor gas absorption. It is known to add from 0.01% to 10% (usually the lower percentages) of an antioxidant, such as epoxidized soybean oil, to the dielectric fluid to increase gas absorptivity. See Canadian Pat. No. 1,044,004 issued to Takashima Kazushige. It is also known that epoxidized soybean oil alone may be used as a dielectric fluid (U.S. Pat. No. 4,422,962 issued to Stanley W. Cichanowski); however, the capacitance loss factor of epoxidized soybean oil is known to be considerably higher than that of glyceryl triacetate, for example 4.2% for the former as compared to 2.7% for the latter.